It has become advantageous to employ high molecular weight polymers of at least one conjugated diene, and particularly high vinyl polybutadiene, in thermosetting compositions designed for injection molding as the highly filled compositions based on these high molecular weight polymers can be processed into a free-flowing form for facile addition to injection molding machines. However, the higher molecular weight conjugated diene polymers often have high compounded viscosities and their moldings sometimes have poor flexural and tensile properties. Various types of fillers have been evaluated as attenuators for these and other problems associated with high molecular weight conjugated diene polymers and their moldings.